Why do you assume marriage means sex ? I know other cultures use and have used marriage as a means of supporting many women. I don't want to use our culture to judge another without full knowledge of what their culture is actually doing.
A quote from the article -
"In a year or 18 months I will marry her off to my youngest son," he said. "He is 19 years old and has been married to his first wife for two years but has not had a child yet."
It certainly sounds like he means his son to have sex with her, don't you think?
In most agricultural societies, a key function of the family is to provide children. Note that the father of the prospective husband of this little girl states that his son's first wife has not born him children. You are kidding yourself if you think that marriage will not lead to sexual relations in this case.